Reaction of Allyl Radical with Acetylene

石油学会誌 J. Japan Petrol. Inst., 23, (2), 133-138 (1980) 133

Daisuke NOHARA* and Tomoya SAKAI*

The title reaction was performed at 580-740℃, employing biallyl as the source of the allyl radical. The main product of addition of allyl radical to acetylene was cyclopentadiene, and the selectivity attained was more than 90 percent of the total C5 products. Such acyclic product as 1,4-pentadiene was scarcely formed in distinct contrast with the result of the addition of allyl radical to ethylene, in which almost equal amounts of 1-pentene and cyclopentene were formed. The product distribution of the title reaction was reported, and a scheme for the product formation was proposed. From the kinetic analysis, the overall activation energy of reaction for the formation of cyclopentadiene from allyl radical and acetylene was estimated.

The present experiment on thermal reaction of 1. Introduction biallyl with acetylene was undertaken to examine Allyl radical has been suggested as playing the the possibility of cycloaddition of allyl radical to role of diene1) in the cyclization reaction with the triple bond. Cyclopentadiene, one of the ex- olefins to form C5-cyclic products analogous to pected C5 compounds in the present reaction, was butadiene which thermally combines with olefins formed in high selectivity, i.e., ca. 40 percent of to yield C6-cyclic compounds. In the pyrolysis all products, including the products from the thermal of ethylene2), C6-cyclic compounds were formed reaction of acetylene itself. The selectivity was mostly through the Diels-Alder reaction between formed to be as high as 60 percent, excluding the butadiene and ethylene, the butadiene being one products from the reaction of acetylene itself. A of the primary products of the reaction. characteristic feature of the reaction of allyl radical with the triple bond was that an acyclic C5 com- ≪+||→〇→-2H2○ pound such as 1,4-pentadiene was scarcely formed. This result was quite different from the result On the other hand, greater amounts of C5-cyclic obtained in the thermal reaction of biallyl in excess compounds were produced in the pyrolysis of pro- ethylene5), where the amount of 1-pentene was as pylene3) than in the case of ethylene. It was abundant as that of cyclopentene. suggested that, in the case of propylene, the difference originated from the contribution of the 2. Experimental allyl radical that was formed through the following The apparatus used was an ordinary atmospheric reaction: flow system. The reactor was made of a quartz annular cylinder. Reactants and products were ◇+◇→◇,◇+H・ analyzed mostly with a FID gas chromatograph equipped with a di-n-butylmaleate capillary column. In our study on the pyrolysis of 1,5-hexadiene Identification of products was confirmed by GC- (biallyl)4), C5-cyclic compounds such as cyclopen- MS. Reaction temperatures were in the range tadiene and cyclopentene were produced as the from 580℃ to 740℃, and residence times were primary products. It is considered that these com- 0.09 to 0.3sec. Equivalent reactor volumes at the pounds were formed by the addition of allyl radical temperatures employed were calculated by the meth- to biallyl. Furthermore, in the pyrolysis of biallyl od of Hougen and Watson6). Acetylene, freed in excess ethylene5), cyclopentene and 1-pentene from acetone vapor, was diluted with deoxygenated were formed with fairly good selectivities, i.e., the nitrogen to around 4vol%. Biallyl vapor was total amount of the two products was ca. 60% mixed with the acetylene-N2 flow and fed into of all products produced. the reactor. Biallyl concentrations were adjusted Received June 18, 1979. * Dept, of Chemical Reaction Engineering, Faculty of around 0.16vol% of the inlet gas. The thermal Pharmaceutical Sciences, Nagoya City University (3-1, reaction of acetylene without biallyl vapor was Tanabedori, Mizuho-ku, Nagoya 467) conducted as a blank test.

石油学会誌 J. Japan Petrol. Inst., Vol. 23, No. 2, 1980 134

(a) (b) Fig. 1 Product Formation Curve in the Thermal Reaction of Acetylene

in terms of apparent activation energy was 110 3. Results and Discussion or 54kJmol-1 without the addition of biallyl; As blank reference for the title reaction, the with the addition of biallyl, however, it changed results and discussion on the thermal reaction of to 150 or 130kJmol-1, respectively. These facts acetylene-N2 will first be briefly described. It will reflect the complexity in the latter case, i.e., the then be followed by a description of the results intermediate radicals in one chain reaction system and discussion of the title reaction of allyl radical affect the reactions of the other system. Common with acetylene. intermediate radicals such as ethynyl and vinyl 3.1 Thermal Reaction of Acetylene are expected to be involved in the chains of both The products obtained in the thermal reaction systems. of acetylene under the present conditions used, 3.2 Reaction of Allyl Radical with Acetylene i.e., ca. 4vol% acetylene in N2 at 580-740℃ Fig. 2 shows the Arrhenius plots for the biallyl for 0.09-0.3sec, were vinylacetylene, benzene and decomposition in the thermal reaction of acetylene- 1,3,5-hexatriene. The presence of the last com- biallyl-N2 gas mixtures that fitted to a first-order pound was indicated from the retention time of rate equation similar to the ethylene case reported glc, but the amount was too small to be identified previously5). The runs conducted at 740℃ (1/T= by GC-MS. 0.99×10-3K-1) were excluded from the figure since The temperatures used in the present experiments were in the lower temperature range classified by Back7) in his study on acetylene pyrolysis where the formation of vinylacetylene and benzene was claimed as the main products7),8). In the lower part of the present temperature range used, polymerization seemed to become predominant7). Rela- tions between product concentrations and residence times are illustrated in Fig. 1 (a). The presence of an induction period was evident as in other reports9),10). The time courses of concentrations of vinylacetylene and benzene with the addition of biallyl vapor are shown in Fig. 1 (b). The induction period was shortened as observed by many investigators who had pointed out the same trend11),12) when another appropriate radical source existed. The temperature dependency of the rate of formation of vinylacetylene or benzene expressed Fig. 2 Arrhenius Plot for the Biallyl Decomposition

石油学会誌 J. Japan Petrol. Inst., Vol. 23, No. 2, 1980 135

Table 1 Typical Experimental Data

○: Cyclopentadiene, ○: Propylene, ●: Ethylene, ○: Butadiene, ○: Butene, ○: 1-Pentene-4-yne, ○: 1,4-Pentadiene, ○: Benzene, ○: Vinylacetylene, ●: 1,3,5-Hexatriene

Fig. 4 Effect of Biallyl Concentration on Molar Amounts of Products ○: Cyclopentadiene, ○: Propylene, ●: Ethylene, ○: Butadiene, ○: Butene, ○: 1-Pentene-4-yne, ○: 1,4-Pentadiene, ○: Benzene, ○: Vinylacetylene, listed in Table 1. ●: 1,3,5-Hexatriene The product distribution of this reaction is shown Fig. 3 Product Distribution in the Reaction of Allyl in Fig. 3. It is clear that the amount of cyclopen- Radical and Acetylene tadiene was ca. 40% of all products, while that of biallyl was completely decomposed at that tempera- 1,4-pentadiene was less than 3%. In the products, ture. From this figure, kd=1012.5exp (-209,000/ excluding vinylacetylene and benzene, about 60% RT)s-1 was obtained. The kinetic parameters were of all products was cyclopentadiene. The present in numerical agreement with those observed in the result contrasted sharply with that observed in the ethylene-biallyl case5), indicating that decomposi- ethylene-biallyl system in which 1-pentene was form- tion of biallyl was unaffected by the presence of ed in the amount nearly equal to that of cyclo- either ethylene or acetylene. pentene5). The striking difference indicates that The main products in the reaction of allyl radical the cyclization of C=C-C-C=C. radical followed with acetylene were cyclopentadiene, propylene, by hydrogen atom elimination to produce ◇ is ethylene, butadiene and 1-butene, in addition to vinylacetylene and benzene; and the minor prod- much faster than hydrogen abstraction by the same ucts were 1,4-pentadiene, 1-pentene-4-yne, 1,3,5- radical to produce acyclic product, presumably, hexatriene and methane. Very small amounts of because of the absence of available H-donors. unidentified products were also detected in the Illustrated in Fig. 4 is the effect of biallyl con- C5 to C6 fractions. Typical experimental data are centration on the product distribution observed at

石油学会誌 J. Japan Petrol. Inst., Vol. 23, No. 2, 1980 136

660℃ and 0.17s. The relative amounts of prod- of this radical eliminates hydrogen to produce ucts, excluding vinylacetylene, benzene and 1,3,5- cyclopentadiene according to Reaction (9). hexatriene, which were formed from the thermal ・◇→◇+H・ (9) reaction of acetylene, increased distinctly with biallyl concentration, indicating clearly that cyclopentadiene was formed through the addition of The hydrogen abstraction product, cyclopentene, allyl radical to acetylene. was not detected in the products. According to 3.2.1 Scheme for Product Formations the above, only small amounts of hydrogen-donating The following scheme is proposed for the reac- compounds, other than propylene, are formed. tions of acetylene-biallyl-N2 gas mixtures. Biallyl This behavior is the characteristic feature of a initially decomposes to produce two allyl radicals system involving acetylene. The H. released in in Reaction (1). Reaction (9) adds to acetylene or other compounds, C=C-C-C-C=C→2C=C-C・ (1) thereby enhancing polymerization and the forma- The fate of the allyl radical thus produced will tion of C2-C4 unsaturated compounds. be its addition to acetylene and to biallyl in Reac- As for the formation of ethylene, butadiene and tions (2) and (3) or the abstraction of hydrogen 1-butene, it was observed that the relationship, to produce propylene in Reactions (4) and (5). ethylene=butadiene+1-butene, was valid for al- C=C-C・+C≡C→C=C-C-C=C・ (2) most the whole region covered by the present

C=C-C・+C=C-C-C-C=C→ experiments, as it was the case of the thermal

C=C-C-C-C-C-C-C=C (3) reaction of biallyl4). This fact implies that,

C=C-C・+C≡C→C=C-C+C≡C・ (4) although biallyl decomposes mostly according to C=C-C・+C=C-C-C-C=C→ Reaction (1), the following type of scission of C=C-C+C=C-C-C-C=C (5) biallyl partly takes place: C=C-C-C-C=C→C=C・+C=C-C-C・ (10) The ethynyl radical formed in Reaction (4) probably enters into a complicated reaction cycle of acetylene From the butenyl or the vinyl radical, 1-butene or undergoes polymerization. The radicals formed and butadiene or ethylene is formed in Reactions in Reactions (3) and (5) also undergo complicated (11), (12) or (13). C=C-C-C・+RH→C=C-C-C+R. (11) polymerizations. Attention is first focused on the 1,4-pentadienyl-1 C=C-C-C・ →C=C-C=C+H・ (12) radical formed in Reaction (2). The fate of this C=C・+RH→C=C+R・ (13) radical will be cyclization and hydrogen abstrac- RH in these Reactions is considered to be acetylene tion. The loss of hydrogen from the 1,4-penta- and/or biallyl. As temperature becomes higher, dienyl-1 radical can also be considered, but it is the formation of 1-butene by hydrogen abstraction not included in our discussion because 1-pentene- becomes more important. Provided that ethylene, 4-yne was scarcely formed in our experiments. butadiene and 1-butene are formed through Reac- Other possible reactions of this radical are the tions (10) to (13), and provided that allyl radical addition to either acetylene or biallyl similar to is converted exclusively to propylene and cyclo- Reactions (2) and (3). pentadiene, the ratio of biallyl decomposition by Reaction (10) to that by (1) will be between 0.15 C=C-C-C=C・ →・ ◇ (6) and 0.25. 3.2.2 Kinetic Studies C=C-C-C=C・+C≡C→C=C-C-C=C+C≡C・ The temperature dependency (TD) of the rate (7) for the cyclopentadiene formation was measured C=C-C-C=C・+C=C-C-C-C=C→ to be 113kJmol-1 in Fig. 5. The TD of the C=C-C-C=C+C=C-C-C-C=C (8) rate for the cyclopentene or 1-pentene formation More than 90% of the total C5 products was cyclo- was measured to be 51.3 or 72.1kJmol-1 by plott- pentadiene, while 1,4-pentadiene amounted only ing the data5) obtained in a previous ethylene- to less than 7%. Reaction (6) is therefore con- biallyl system as shown in Fig. 6. The TD was siderably faster than the combined total of Reactions much larger for the cyclopentadiene formation than (7) and (8). that for the cyclopentene formation. The rate of The cyclopentenyl radical formed in Reaction the cyclopentadiene formation at 700℃, however, (6) either releases or abstracts hydrogen. Most was 200 times larger than that of the cyclopentene

石油学会誌 J. Japan Petrol. Inst., Vol. 23, No. 2, 1980 137

Fig. 6 Temperature Dependency of Cyclopentene or 1-Pentene Formation

Fig. 5 Temperature Dependency of the Rate of reaction, could not be deduced from the kinetic Cyclopentadiene Formation analysis of the present data. formation, when a comparison was tentatively made References at the same concentration levels of acetylene or 1) Bryce, W. A., Ruzicka, D. J., Can. J. Chem., 38, 835 ethylene and biallyl. A large pre-exponential (1960). factor of the acetylene-biallyl reaction is the cause 2) Kunugi, T., Sakai, T., Soma, K., Sasaki, Y., Ind. for this result. Eng. Chem., Fundamentals, 8, 374 (1969). In a previous kinetic analysis made on the dial- 3) Kunugi, T., Sakai, T., Soma, K., Sasaki, Y., Ind. Eng. Chem., Fundamentals, 9, 314 (1970). lyloxalate pyrolysis in excess ethylene13), TD's of 4) Nohara, D., Sakai, T., Ind. Eng. Chem., Prod. Res. k-values for cyclopentene and 1-pentene formations Develop., 12, 322 (1973). were obtained as 48.0 or 69.6kJmol-1, instead 5) Sakai, T., Nohara, D., Bull. Japan Petrol. Inst., 17, of 51.3 or 72.1kJmol-1 which is the TD for the (2) 212 (1975). 6) Hougen, D. A., Watson, K. M., "Chemical Process rate of cyclopentene or 1-pentene formation in Principles", 884 (1943), Wiley, New York, N. Y. the ethylene-biallyl case. Accordingly, the TD of 7) Back, M. H., Can. J. Chem., 49, 2199 (1971). the concentration of allyl radical within the range 8) Cullis, C. F., Franklin, N. H., Proc. Roy. Soc. A, 280, from 580 to 700℃ is apparently obtained as 51.3- 139 (1964). 9) Minkoff, G. J., Newitt, D. M., Rutledge, P., J. Appl. 48.0=3.3kJmol-1 from the cyclopentene formation Chem., 7, 406 (1957). or 72.1-69.6=2.5kJmol-1 from the 1-pentene 10) Silcocks, C. G., Proc. Roy. Soc. A, 242, 411 (1957). formation. As the same temperature region was 11) Cullis, C. F., Minkoff, G. J., Nettleton, M. A., Trans. applied to the present reaction, the TD of the Faraday Soc., 58, 117 (1962). k-value for the acetylene-biallyl case could be esti- 12) Rice, F. O., Walters, W. D., J. Am. Chem. Soc., 63, 1701 (1941). mated as 113-(3.3+2.5)/2=110kJmol-1. How- 13) Sakai, T., Nohara, D., Kungi, T., "ACS Symp. Ser., ever, the estimation of the A-factor for the k-value, Industrial and Laboratory Pyrolyses", 32, 152 (1976). inspite of its importance in the acetylene-biallyl

石油学会誌 J. Japan Petrol. Inst., Vol. 23, No. 2, 1980 138

要旨

アリルラジカルとアセチレンの反応

野原大輔*, 酒井朝也*

ビアリルをアリルラジカル生成源として用い, アリルラジカ実と大いに異なり, アリルラジカルのアセチレンへの付加におルとアセチレンの反応を行った。反応装置は常圧流通式のものいては環状化合物の生成速度が非常に大きいことを示していであり, ビアリルおよびアセチレンをN2中それぞれ約0.16 る。また, ビアリル蒸気の添加量を変化させ, 他は同一条件におよび4mol%の濃度とし, 反応温度は580～740℃, 滞留時保って行った反応の結果をFig. 4に示した。ビニルアセチレ間は0.09～0.3secであった。ン, ベンゼン等アセチレン単味の反応生成物以外の生成物は明ブランクとしてビアリル蒸気を添加しないアセチレン-N2混らかにビアリル濃度に依存していることが示されている。合ガスの実験も行った。この反応の主生成物はビニルアセチレ今回の反応におけるシクロペンタジエン生成速度の温度依存ンおよびベンゼンであった。これらの生成曲線を, ビアリル蒸性をFig. 5に示し113kJmol-1を得た。また, 前回行った気を添加した場合のものと並べてFig. 1に示した。ビアリルとエチレンの反応におけるシクロペンテンおよび1- アリルソースとしてのビアリルの分解反応は一次反応としてペンテンの生成速度の温度依存性をFig. 6に示した。これに整理でき, その結果をFig. 2に示した。より, 51.3および72.1kJmol-1を得た。ビアリルオギザレー本反応の実験結果の一部をTable 1に, 生成物分布をFig. ト (DAO) をアリルラジカル生成源として用いたDAOとエ 3に示した。アリルラジカル付加生成物として検出されるC5 チレンの反応により, アリルラジカルとエチレンからシクロペ化合物の中で, シクロペンタジエンが90%以上という高い選ンテンおよび1-ペンテンが生成する2次速度定数の温度依存択性をもって生成した。一方, 対応して生成されると考えられ性はそれぞれ48.0および69.6kJmol-1と求まっている。る非環状のC5化合物, 1,4-ペンタジエンは極く少量しか存在 51.3および72.1kJmol-1との差はアリルラジカル濃度の温しなかった。この結果は前に行ったビアリルとエチレンの反応度依存性分である。これを考慮して, アリルラジカルとアセチにより, 1-ペンテンがシクロペンテンとほぼ等モル生成した事レンの付加によるシクロペンタジエン生成の2次速度定数の温 * 名古屋市立大学薬学部 (467 名古屋市瑞穂区田辺通 3-1) 度依存性を110kJmol-1と推算した。

Keywords Allyl radical, Acetylene, Cycloaddition, Cyclopentadiene, Kinetics

石油学会誌 J. Japan Petrol. Inst., Vol. 23, No. 2, 1980